Threaded joints for pipes are popularly used to connect steel pipes used in petroleum installations of oil industry such as oil country tubular goods. In connecting steel pipes used for search and production of oil or gas, conventionally, a standard threaded joint stipulated in API (American Petroleum Institute) standard has been typically used.
However, recently, since the deep wells for crude oil or natural gas have been in progress and horizontal wells and directional wells on behalf of vertical wells have been increasing, excavation and production environments are becoming severe. Further, wells developed in severe circumstances such as oceans and polar regions have been increasing. Hence, the performances that threaded joints have to satisfy are diversified including compression resistance, bending resistance, sealability against external pressure (external pressure resistance). In view of the above, the use of a special threaded joint having high performances which is referred to as a “premium joint” is increasing, and a demand for the improvement in performance of the premium joint is also steadily increasing.
The premium joint is ordinarily a coupling-type joint. The coupling-type joint joins an externally-threaded member (hereinafter referred to as “pin”) formed on a pipe end portion and an internally-threaded member (hereinafter referred to as “box”) to connect the pins to each other. The pin includes a tapered thread, a metal-to-metal seal portion (hereinafter referred to as a “seal portion”) and a torque shoulder (hereafter referred to as a “shoulder”). The box also includes a tapered thread, a seal portion and a shoulder. The tapered thread is important to firmly fix the pipe joint. Bringing the box and the pin into a metal contact at seal portions thereof has the seal portions function to ensure sealability. The shoulder forms a shoulder surface that functions as an abutment during making up the joint.
FIG. 3(a) to FIG. 3(c) are schematic explanatory views of a premium joint for oil country tubular goods, which are vertical cross-sectional views of a threaded joint for cylindrical pipes. The threaded joint includes a pin 3 and a box 1 into which the pin 3 is fitted. The pin 3 has, on the outer surface thereof, an externally-threaded member 7 and a nose (also referred to as a pin nose) 8 which is a portion formed adjacent to the externally-threaded member 7 on an end of the pin 3 and has no threads. The nose 8 has a seal portion 13 on the outer peripheral surface thereof, and a shoulder 14 on the end surface thereof. The box 1 into which the pin 3 is fitted is a portion having an internally-threaded member 5, a seal portion 11 and a shoulder 12 on the inner surface thereof, and these portions 5, 11, and 12 are portions being threadedly engaged with or brought into contact with the externally-threaded member 7, the seal portion 13 and the shoulder 14 of the pin 3, respectively. In FIG. 3(a), a chain line indicates a pipe axis.
In the conventional example shown in FIG. 3(c), the threaded joint is of a so-called “pin end seal” type where the seal portions 11 and 13 are positioned at the end portion of the nose 8, thereby imparting an appropriate make up torque to realize desired seal performance. However, in the conventional example shown in FIG. 3(c), the make up torque is influenced by lubrication condition, surface quality and the like. In view of the above, as a design of a threaded joint that does not largely depend on these factors, there has been known a radial-directional seal type (also referred to as a radial seal type) threaded joint where a radial-directional component of seal contact pressure is relatively increased. The seal portions 11 and 13 of the radial seal type threaded joint are generally positioned at an intermediate portion of the nose 8 in the pipe axis direction.
Along with the progress of deep wells described above, the use of heavy-walled oil country tubular goods having high corrosion resistance and high strength has increased to cope with a high-temperature and high-pressure environment.
Under such circumstances, there is a related art to cope with a demand for a threaded joint having high galling resistance and high sealability even under a large compressive load while maintaining gas tight performance of the threaded joint per se. For example, a means where a ratio of a cross-sectional area of contact surface of shoulder on a box side (an area of region obtained by projecting the contact surface on a plane orthogonal to a pipe axis) to a cross-sectional area of a pin raw pipe (a cross-sectional area of a pin non-formed portion of steel pipe where pin forming is applied to an end portion of the steel pipe) is set to a predetermined value (0.5) or more (Japanese Patent No. 4257707 (JP-T-2005-526936)) is known.
Particularly, a threaded joint including a pin where a wall thickness of a raw steel pipe for forming the pin (wall thickness of a non-formed portion of the steel pipe where pin forming is applied to an end portion of the steel pipe) is 18 mm or more and a box that connects the pin to each other, needs ensuring sealability under a composite load of extremely high axial compressive force and extremely high external pressure. Hence, a seal portion is required to possess a high contact pressure. On the other hand, when the contact pressure is excessively high, there is a possibility that galling occurs due to sliding at the time of making and breaking the threaded joint. Accordingly, it is necessary to design a contact pressure such that both ensuring sealability and prevention of galling are well-balanced.
However, the above-mentioned related art expects only the ensuring of performances under a high compressive force, and does not take into consideration acquisition of both ensuring sealability and prevention of galling in a state where a high external pressure is also applied in addition to a high compressive force. According to the above-mentioned related art, it is impossible to acquire both ensuring sealability and prevention of galling in a state where a high external pressure is also applied in addition to a high compressive force by only setting of a ratio of a cross-sectional area of contact surface of shoulder on a box side to a cross-sectional area of a pin raw pipe to a predetermined value (0.5) or more.
It could therefore be helpful to provide a threaded joint for heavy-walled oil country tubular goods that both ensures sealability and prevents galling in a state where a high external pressure is also applied to the threaded joint in addition to a high compressive force.